1. Field of the Invention
The present invention relates to a method for preventing the formation of hydrolysis products of fluorine-containing gas streams, such as uranium hexafluoride (UF.sub.6) and/or gaseous fluorination agents, in industrial systems handling such gas streams in which fluoroelastomers are conjointly used in articles, such as seals.
2. Background of the Art
The use of commercially available fluoroelastomers, such as those bearing the trade names Viton.RTM. and Technoflon.RTM., marketed respectively by E. I. Dupont de Nemours of Wilmington, Del. and Montecatini of Milan, Italy, as sealing materials in plants handling UF.sub.6 is known. Fluoroelastomeric seals and related flluoroelastomeric articles are used to isolate the process gas of an industrial system (for example, UF.sub.6 alone or with additional gases) from the atmosphere, as well as internally, for example, in isotope separating systems to maintain separate gas streams of different UF.sub.6 concentrations and isotope concentrations, respectively.
The great drawback of previously employed fluoroelastomers is that constituents thereof react chemically even at room temperature with the hydrogen fluoride (HF) which is always present in industrial UF.sub.6, and decompose the HF while forming water. Liberated water then reacts with UF.sub.6 and decomposes the UF.sub.6 while forming HF, resulting in a cyclically continuous decomposition of the UF.sub.6.
Stated more precisely, additives in the form of highly basic materials are required for the manufacture of fluoroelastomers and include oxides, hydroxides and/or carbonates of alkaline earth metals and/or similar oxygen-containing compounds of the Group II metals. These additive compounds are capable of chemically binding with, i.e., reacting with, the HF in the UF.sub.6 gas. In this reaction, water (H.sub.2 O) is formed according to the following schematic equation: EQU Me(OH).sub.2 +2HF.fwdarw.MeF.sub.2 +2H.sub.2 O (1)
where Me represents an alkaline earth metal. This H.sub.2 O then reacts with UF.sub.6 in the surrounding gas phase according to the following equation: EQU UF.sub.6 +2H.sub.2 O.fwdarw.UO.sub.2 F.sub.2 +4HF (2)
The HF produced thereby is able to form H.sub.2 O again according to the reaction of equation (1) so that a continuous UF.sub.6 decomposition may undesirably develop.
The UO.sub.2 F.sub.2 from the reaction of equation (2) precipitates in the form of a dust (hereinafter referred to as hydrolysis dust) which, sooner or later, deposits in sensitive regions, e.g., in the partition structures of isotope separating systems, such as gas diffusion systems, separating nozzle systems, and/or gas ultracentrifuge systems. Such deposits may partially clog and/or change the mechanical characteristics of these sensitive regions.
To avoid these drawbacks, metallic seals have been used instead of elastomeric seals. The use of metallic seals, however, sometimes requires complicated structural modifications to enable the metallic seals to absorb the required high deformation forces. Moreover, metallic seals are more expensive than elastomeric seals.